The present invention relates to a wheel alignment technique. In particular, the present invention relates to an improved technique for measuring caster and camber during wheel alignment.
It is important that wheels of an automotive vehicle are properly aligned in order for the wheels of the vehicle to wear properly and in order for the vehicle to handle properly. In addition, proper alignment of the wheels decreases fuel consumption and increases vehicle safety. The wheel alignment parameters which are typically measured and adjusted in order to achieve proper alignment are camber, caster, steering axis inclination (SAI), pitch and toe.
A camber angle is the inclination of the wheel plane with respect to vertical. It is considered positive when the wheel leans outward at the top and it is considered negative when the wheel leans inward. Any wheel of an automobile can have camber.
A caster angle is the angle in side elevation between the steering axis of a steerable wheel (typically one of the front wheels) with respect to vertical. The caster angle is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward.
Steering axis inclination (SAI) or king pin inclination is the angle in the front elevation between the steering axis and vertical.
Pitch is an angle which measures rotation of a sensor in the plane of a wheel. Pitch angle is measured relative to ground.
The static toe angle, at a specified wheel load or relative position of the wheel center with respect to the sprung mass, is the angle between the longitudinal axis of the vehicle and the line of intersection of the wheel plane and the road surface. The wheel is said to be "toed-in" if the forward portion of the wheel is turned toward a central longitudinal axis of the vehicle. The wheel is "toed-out" if the forward portion of the wheel is turned away from the central longitudinal axis of the vehicle.
Camber, caster, SAI, and pitch are typically measured using one or more inclinometers which are attached to the wheels. In order to measure camber, the inclinometer measures the angle that the plane of the wheel makes with the vertical. This is similar to the pitch measurement. To measure caster, the wheel is turned through an arc (or a "caster swing" is performed), and the difference between the camber readings is used to derive the caster value. SAI is measured in a manner similar to caster, except that the inclinometer used for the SAI reading measures the change in inclination angle of a line in the plane of the wheel as the wheel is turned through an arc. The SAI measuring inclinometer is aligned at 90 degrees to the inclinometer used for reading camber and caster.
From the above discussion it is seen that caster is not an alignment angle that is directly measurable on most vehicles. Rather, the caster swing is necessary to obtain a measure of a vehicle's caster. The same holds true for SAI. In order to obtain a current, or live, reading for caster, a live caster adjustment meter is provided. To use the meter properly, the operator must make certain that the toe of the wheel being measured is zero degrees plus or minus a small tolerance. Thus, a "toe zero" meter is provided along with the live caster adjustment meter to indicate when the toe of the wheel being measured is within the tolerance allowed.
Previous alignment systems use the following formula for live caster: EQU live caster=live pitch-pitch at zero toe+Calculated Caster Eq. 1
In this formula the live pitch is a live reading measured from pitch sensors. Pitch at zero toe and Calculated Caster are constants. The pitch at zero toe is obtained at the end of the caster swing by zeroing the toe on each wheel and then reading the pitch. This is a time consuming step because the toe must be set very precisely. This step is eliminated in the new invention. The Calculated Caster is determined using formulas in Kline et al. U.S. Pat. No. 5,014,227.
Caster affects the camber readings in the same way that SAI affects the pitch readings. On most vehicles, the caster angle is much less than the SAI. Therefore, the effects of caster on camber are less noticeable if the toe of the wheel being measured is non-zero than are the effects of SAI on caster. However, for a more accurate camber reading, it is desirable to substantially minimize the effects of caster on the camber reading. Typically, in order to minimize those effects, the toe of the wheel being measured would be placed at zero plus or minus a very small tolerance.